In many electronic and electrical systems, a large capacitive load is connected in parallel with the system load to filter out current spikes and noise on the power-supply lines. If unfiltered, these spikes and noise can damage the system load, the power supply, or both. During power-up, a large current can flow through the capacitive load, charging the constituent capacitors. This current is commonly known as “in-rush” current and it can substantially exceed the magnitude of the peak current of the electronic or electrical system during its normal operation. Thus, a large in-rush current can damage the power source or may disrupt the operation of other devices connected to the same power bus.
The in-rush current can be limited by connecting passive elements, such as an inductor or a temperature-sensitive resistor, in series with the power source and the system load. Active elements, such as a transistor, may also be connected in series with the power source and the load to limit the in-rush current. The active element(s) may be controlled either by an external source or by a feedback circuit that senses the total current flowing from the power source.
Systems using passive elements tend to be bulky, as the passive elements used are large in size. They also respond relatively slowly to a change in the supply voltage and, hence, may be unsuitable when the supply voltage can change frequently in a short time period. In-rush current limiting systems using active components may require an additional power supply. Moreover, the active element in these systems is connected in series with the power source and the load. As a result, the load current flows through the active element during normal course of operation of the system (i.e., beyond the initial power-up period), causing the system to dissipate power during its normal operation.